The present invention relates to a flexible pipe for transporting a petroleum fluid such as a multiphase fluid or a gas. More particularly it relates to a flexible pipe of the unbonded type for transporting hydrocarbon gases or biphasic hydrocarbons including a gaseous phase.
The invention also relates to a method for testing the seal of an interlayer space between two sheaths of such a flexible tubular pipe.
These flexible pipes, which are formed by an assembly of different concentric and superposed layers, are said to be of the unbonded type as soon as these layers have some freedom to move relatively to each other. These flexible pipes La. meet the recommendations of the normative documents API 17J <<Specification for Unbonded Flexible Pipe>>and API RP 17B<< Recommended Practice for Flexible Pipe>> established by the American Petroleum Institute.
Flexible tubular pipes include at each end, a connecting endpiece intended to ensure the connection of the pipes with each other or with terminal equipment and these endpieces should be made under conditions ensuring good firm attachment and a good seal.
The constitutive layers notably comprise polymeric sheaths generally providing a sealing function, and reinforcement layers intended for absorbing mechanical forces and formed with windings of sheets, metal wires, various strips or profiles in composite materials.
The flexible pipes of the unbonded type which are the most used in the off-shore petroleum industry generally comprise from the inside to the outside, an internal carcass consisting of a interlocked strip which is used for preventing the crushing of the pipe under the effect of the external pressure, at least one polymeric sealing sheath, a pressure vault consisting of at least one shaped metal wire, interlocked and helically wound with a short pitch, said pressure vault being used for absorbing radial forces related to the internal pressure, tensile armor layers formed with long-pitch helical windings of composite metal wires, said armor layers being intended for absorbing longitudinal forces to which the pipe is subject, and finally an external sealing sheath intended to protect the reinforcement layers from seawater.
When the pipe is intended for conveying hydrocarbons at a high temperature, typically between 100° C. and 130° C., the polymer seahg sheath, further called a pressure sheath, is generally made by extrusion of a polymer based on PVDF (polyvinylidene fluoride). This type of polymer however has the drawback of not being very ductile at low temperature so that it is necessary to ensure that the pressure sheath has good surface condition in order to avoid the fatigue breakage phenomenon under the effect of curvature variations. This is why in certain dynamic applications, a second polymeric sheath, a so-called sacrificial sheath, is interposed between the internal carcass and the pressure sheath in order to protect this pressure sheath against the roughnesses of the internal carcass. In this way, if a crack is initiated at the interface between the sacrificial sheath and the internal carcass, and then propagates until it passes right through the sacrificial sheath, it is stopped by the interface between the sacrificial sheath and the pressure sheath, so that the pressure sheath is protected.
A problem resulting from the transport of petroleum fluids at high temperature and pressure is related to the permeability of the polymeric sheaths. Indeed, diffusion of gas through the thermoplastics used for making the sealing sheaths increases with temperature and pressure.
Thus, during the transport of this type of high pressure and high temperature fluids, acid compounds such as hydrogen sulfide H2S and carbon dioxide CO2, as well as steam tend to migrate through the polymeric sheaths until they reach the metal portions of the pipe and risk causing corrosion and embrittlement phenomena by hydrogen. These corrosion and embrittlement phenomena of the metal portions may affect the mechanical integrity of the pipe which is strongly stressed by the high pressures of the transported petroleum fluid and by the marine environment. This is why the metal layers are then made with special corrosion-resistant steels, but such steels have the drawbacks of being expensive or of having poor mechanical characteristics, which imposes an increase in the section and the weight of the metal portions.
Moreover, during the transport of multiphase fluids, diffusion of the gases through the polymeric sheaths causes an increase in the pressure around these sheaths. When this pressure becomes greater than the internal pressure prevailing in the pipe, such as for example during decompression following production stoppage, the pressure difference may lead to the collapse of the internal carcass. This diffusion phenomenon not only concerns the aforementioned gases, i.e. H2S, CO2 and steam, but also methane CH4 which is generally present in a large proportion and under high pressure in the transported hydrocarbons, and the diffusion of which may consequently generate significant mechanical problems.
A solution for avoiding collapse of the internal carcass consists of increasing the thickness of the strip making up this carcass.
But this overdimensioning is expensive and especially has the drawback of increasing the weight of the pipe.
In order to avoid the corrosive effects of the acid compounds such as H2S and/or CO2, it is known for example from documents EP 844429 and WO 2009/153451, how to introduce into a polymeric sheath, products which are chemically active towards said acid compounds so as to neutralize them. These chemically active products may be introduced into the sheath as particles or deposited at the surface.
But, the incorporation of this type of product in polymeric sheaths increases the manufacturing costs of flexible pipes equipped with these sheaths.
The goal of the invention is to find a remedy to the aforementioned drawbacks of the structures of the prior art by proposing a flexible tubular pipe which may be used for transporting under great pressure and high temperature, multiphase petroleum fluids or gases.